1. Field of the Invention
The invention relates to a coating apparatus and to a process for the plasma coating of workpieces, in particular a coating apparatus having a plurality of coating places and a process for the simultaneous coating of a plurality of workpieces.
2. Description of Related Art
The barrier properties of plastic containers, such as for example plastic bottles can be improved considerably by barrier layers on the inner surface or outer surface of such containers. It is in this way possible, for example, to protect foodstuffs from the effects of oxygen, which is relatively good at diffusing through most types of plastics.
Coatings of this type can be deposited on the container walls, inter alia by means of various CVD processes (CVD: chemical vapor deposition). For this purpose, the plasma is generally ignited at a low pressure in a gas atmosphere which includes a precursor gas. The reaction products formed in the plasma are then deposited as a coating on the workpiece to be treated. The low-pressure atmosphere required to ignite the plasma needs the area surrounding the workpiece to be evacuated. This can be done, for example, by suitable lock means or also by the workpiece being introduced into the reactor space under standard pressure and the reactor space then being evacuated. The way in which the workpiece is transferred from a standard pressure atmosphere to a low-pressure atmosphere or a vacuum is accordingly a key problem with regard to the process speed and costs for a CVD coating.
WO 01/31680 A1 has disclosed an apparatus for the low-pressure plasma treatment of containers in which the treatment station includes a stationary cavity which can be closed and opened by means of a removable cover, the cover having a connecting passage which, in the closed state of the treatment station, produces a connection to a vacuum circuit. The containers which are to be coated are inserted into the stationary cavity and then the cover is closed, after which the cavity can be evacuated.
However, this design is disadvantageous in that the container which is to be coated has to be conveyed into the cavity along two mutually perpendicular directions, which involves a complicated movement. Moreover, when inside the cavity the container is surrounded on all sides, apart from the opening which is covered by the cover in the closed state, by the walls of the cavity and is therefore difficult to grip after coating has taken place in order to be conveyed onward. Furthermore, the throughput which can be achieved through an apparatus of this type is limited, since the sequence of movements has to be repeated for each individual container for each coating operation.